This invention relates to a walking beam furnace the hearth of which consists of alternately-disposed fixed beams and walking beams, wherein the walking beams have associated therewith lifting rollers which are mounted on a lifting frame and which run on ramps that are prolonged downwards by a longer distance than is required for the working stroke or lift thereof.
A furnace of this type is illustrated in the U.S. Pat. No. 4,330,262. In this furnace, the walking beams can be lifted, together with the articles to be heated and by means of lifting rollers running on the ramps, beyond the hearth of the furnace and then moved forwards, whereupon the walking beams are lowered again below the hearth of the furnace and moved backwards to the starting position thereof. Thus, the articles to be heated can be advanced step by step through the heating furnace. The ramps whereon the lifting rollers run are prolonged downwards to such an extent permitting the walking beams to be lowered to a lowermost position wherein both the walking beams and fixed beams can be reached for repair and maintenance purposes. The lifting rollers can be moved by an extensible lifting rod which is pivoted to a lifting frame mounting the lifting rollers. Said lifting rod is fixed to an oscillating arm which is pivoted on a support stand and is moved by an actuating cylinder. In order to depress the walking beams into their maintenance position, the lifting frame is moved by an extent corresponding to the stroke of the piston of said cylinder and is then locked against any further movement. The lifting rod is then detached from the oscillating arm and the piston is moved back to its other end position. Thereafter, an extension rod is inserted between the lifting rod and the oscillating arm, and the lifting frame is then unlocked. Thereafter, the actuating cylinder is activated again so as to cause a further downward movement of the walking beams. Thus, the lifting rod is extended step by step and the lifting rollers will run farther and farther downwards on the ramps. The walking beams then follow this lowering movement correspondingly.
By pivotally connecting the lifting rod at a plurality of pivot points established at different distances from the fulcrum of the oscillating arm, a plurality of different drive-ratios are obtained between the movement of the actuating cylinder and the movement of the lifting rod and, therefore, walking beams. However, said drive-ratio can be changed only within a narrow range, i.e. about 2:1, because a compromise is necessary between the operative stroke or lift and the depression stroke into the repair position, when the latter stroke is to be performed by a minimum of steps. The locking of the lifting rod and the addition of each extension rod after each lowering step require additional labor, particularly when the lowering into the repair position requires a great number of individual steps. This disadvantage is not very serious in case of small walking beam furnaces because the weight difference between a loaded walking beam effecting its operative lift and an unloaded walking beam while being lowered into its maintenance position is not very great.
However, in case of larger walking beam furnaces, said weight difference is considerable and requires that the lowering stroke be composed of a great number of individual steps, or that the actuating device for the walking beam be much more powerful.
Inasmuch as only the operative lift requires a great power, whereas the successive lowering or depression stroke to reach the maintenance position is effected practically with no load, the invention aims to improve the walking beam furnaces described in the preamble so that, while maintaining their constructional simplicity and minimizing the costs, the operative lift shall require a relatively small actuating force and the lowering stroke to the maintenance position shall be effected at a faster rate.
The solution of this problem is based on a variable-ratio drive for the actuation of the walking beams. Particularly, the invention comprises a drive-transmitting member in the form of a control rod or, preferably, a sprocket chain or a cable, which is pivoted at one end to an end of the lifting frame and at the other end to a wheel which is rotatably mounted around a fixed axis. This wheel is rotated by a hydraulic, pneumatic or electric mechanism which is pivotably connected at one end to said wheel and at the other end to a reaction-bearing support.
The long and heavy oscillating arm is thus eliminated, and the walking beams can be stopped in any position by locking said wheel. For this purpose, a supporting stand for the wheel can be provided with holes co-operating with a plurality of registering holes spaced around the periphery of the wheel, and suitable locking pins can be inserted thereinto. The holes spaced around the periphery of the wheel also serve for connecting to different points of the wheel a pivotal eye of the actuating cylinder assembly, by means to a removable pivot pin, whereby with a relatively small stroke of the actuating assembly, a step by step rotation of the wheel is obtained over half of a round angle, or even more.
On the other hand, the ramps may comprise, as is known in the art, a less steep stretch or length for the operative stroke or lift, and a more steep stretch or length for the lowering or depression stroke to the maintenance position. This construction of the ramps is advantageous, because the operative stroke or lift is smaller than the lowering stroke into the maintenance position, and because the effort at the less steep stretch of the ramp is (even under heavy loads) much smaller than that required at a more steep stretch, such as that for the lowering stroke into the maintenance position.
When the ramps for the lifting rollers comprise a less steep stretch and a more steep stretch, but also when the ramps are continuous and uniform, a variable-ratio drive for actuating the walking beams can be obtained according to the invention by means of a drive-transmitting member which is pivoted to the wheel so as not to rest on the peripheral surface of the wheel. When said drive-transmitting member is formed by a control rod connected to the lifting frame, said rod is pivoted to the wheel in such a way that, to perform the operative lift, a rotation of the wheel corresponding to the stroke of the actuating assembly causes only a small translational movement of the control rod and, therefore, a small lift of the walking beams, whereas the lowering stroke to the maintenance position corresponds to an almost tangential movement of the control rod with respect to the wheel and, therefore, to a correspondingly longer stroke.
When the drive-transmitting member is formed by a flexible member, for example a cable or a sprocket chain, such a member is pivoted to the wheel as described above and rests, during the operative lift, on a support member co-axial with the wheel and having a diameter smaller than the wheel. During the operative lift, the drive-transmitting flexible member rests on said smaller-diameter support member and the movement of the lifting frame is a function of the radius of this support member. When the wheel is rotated step by step by the actuating assembly to such an extent that the drive-transmitting member is raised from said smaller-diameter support member, the movement of the lifting frame is increased by an extent corresponding to the distance between the centre of the wheel and the taut flexible drive-transmitting member, thus initiating the lowering stroke to the maintenance position.
When the drive-ratio between the actuating assembly and the lifting frame is to be unchanged, the wheel is so constructed as to permit the drive-transmitting member to rest on the peripheral surface of the wheel. In this instance, the operative lift and the lowering stroke to the maintenance position can be differentiated from each other only by the differently-inclined stretches of the ramps.